(a) Field of the Invention
The invention relates to a transflective liquid crystal display device and, more particularly, to a transflective liquid crystal display device using an OCB (optically compensated birefringence) technique which realizes a wide view-field angle and high-speed response.
(b) Description of the Related Art
Twisted nematic (TN) cells, which at present are widely used in thin film transistor (TFT) liquid crystal display devices, have a small view-field angle and low-speed respond. To solve this problem, studies on an OCB (optically compensated birefringence) technique have been made. Specifically, in case of displaying moving images, a response time is required in the order of ten milliseconds or even less, and the OCB technique is possible to obtain a high-speed response characteristic as well as a wide view-field angle. Hence, it is the best mode suitable for accommodating a recent trend of multimedia and motion picture processing that allows a vast amount of image data to be moved on the screen of a display device at high speed.
FIGS. 1A-1C show schematic diagrams illustrating the transformation of splay-bend alignment of liquid crystal molecules in an OCB cell 102.
Referring to FIG. 1A, the liquid crystal layer exhibits an initial splay-aligned state when no voltage is applied across it, where the liquid crystal molecules in the region A and region B tilt at a very small angle (compared with the later described angles shown in FIG. 1B and FIG. 1C) to the alignment films 104 and 106. Also, the liquid crystal molecules in the region C turn to nearly parallel to the alignment films 104 and 106.
Next, as shown in FIG. 1B, when a voltage is applied to the liquid crystal layer exhibiting splay alignment and gradually increased to approach a threshold voltage Vt, the splay alignment of liquid crystal molecules is deformed and transformed to a bend alignment, where the angle between the liquid crystal molecules and the alignment film 104 in the region A and the angle between the liquid crystal molecules and the alignment film 106 in the region B are both increased. Further, the liquid crystal molecules in the region C are almost perpendicular to the alignment films 104 and 106. At this time, the OCB cell is in a white state and has maximum brightness.
Then, when the voltage is continually increased to exceed a critical voltage Vc, the angle between the liquid crystal molecules and the alignment film 104 in the region A, and the angle between the liquid crystal molecules and the alignment film 106 in the region B are both increased to a maximum, about 80 degrees or more. Further, most of the liquid crystal molecules in the region C are perpendicular to the alignment films 104 and 106. At this time, the aligned direction of the liquid crystal molecules is almost perpendicular to the absorption axis of a polarizer (not shown), so the OCB cell is in a dark state. Thus, the OCB mode works when the applied voltage has a value between the threshold voltage Vt and the critical voltage Vc.
In the OCB cell, the liquid crystal molecules are well arranged to allow for a reduced friction during rotation, so that a wide view-field angle and high-speed response are realized. However, in order to operate an OCB mode LCD device, the applied voltage should be higher than the threshold voltage Vt. Thus, it may take much time to increase the applied voltage from zero to reach a threshold value once the threshold voltage Vt is high. More specifically, the threshold voltage Vt should be as low as possible to decrease the response time of the OCB mode LCD device and to prevent its thin film transistors from being damaged.
On the other hand, the OCB technique is typically used in either a transmission type or a reflection type LCD device. Though the transmission type LCD device uses backlight to obtain a bright display independent of surrounding environments, the panel brightness is often not sufficient when the device is exposed to direct sunlight. In comparison, a reflection type LCD device employs surrounding light to effect a display so that a backlight source is omitted; however, the reflection type LCD device is largely deteriorated in visibility in a dark surrounding. Hence, there has been a strong demand for designing an OCB mode LCD device that operates at a low threshold voltage and possesses good visibility in any environment.